Key points:

There is no prospect of further sanctions on Russia being agreed on Monday

Ms May could present her case for any such measures at an EU summit on Thursday

Boris Johnson, who briefed fellow European Union ministers in Brussels, also won renewed support from the bloc, though diplomats cautioned there was no immediate prospect of fresh economic sanctions on Russia.

“The European Union takes extremely seriously the UK Government’s assessment that it is highly likely that the Russian Federation is responsible,” the statement said.

Swedish Foreign Minister Margot Wallstrom rejected an allegation by the Russian Foreign Ministry that the nerve agent used in Salisbury might have come from Sweden.

“This is just ridiculous and totally unfounded,” Ms Wallstrom said.

“I think they are trying to divert the real issues here.”

While there is no prospect of further sanctions on Russia being agreed on Monday, British Prime Minister Theresa May will have an opportunity to present her case for any such measures at an EU summit on Thursday, or call for others to expel diplomats.

“We need to put pressure on Russia to take part in a real enquiry about the attack,” Belgian Foreign Minister Didier Reynders told reporters.